Abstract: Comparing several kinds of resonators with annular gain media, the new type of multipass resonator with two annular spherical mirrors which uses an entire working surface output is considered suitable for a kind of high power CO2 laser. A new method for calculating the optic field distribution at the mirror of coaxial discharge laser with annular gain media is proposed. An integral area is obtained using solid geometry based on Fresnel-Kirchoff diffractive integral equation, which is divided into finite grids. Then the optic field distribution at the mirror of annular resonator is calculated. Near-field distribution in the middle of the annular area is similar to the Gaussian distribution, and the high-order mode distribution appears along the radius. Azimuthal distribution has no discrim